<!DOCTYPE html>
<html>
<head>
    <meta http-equiv="Content-type" content="text/html; charset=utf-8" />
    <title></title>
    <style>
        div {
            width: 100px;
            height: 90px;
            float: left;
            padding: 5px;
            margin: 5px;
            background-color: #EEEEEE;
        }
    </style>
    <script src="http://libs.baidu.com/jquery/1.9.1/jquery.js"></script>
</head>

<body>   
     <div id="aaron">
        <p>id="aaron"</p>
        <p>选中</p>
    </div>
    <div id="imooc">
        <p>id="imooc"</p>
        <p>jQuery选中</p>
    </div>
    <div id="imooc">
        <p>id="imooc"</p>
        <p>jQuery未选中</p>
    </div>

    <script type="text/javascript">
       //通过原生方法处理
        var div = document.getElementById('aaron');
        div.style.border = "3px solid blue";
    </script>

    <script type="text/javascript">
        //通过jQuery直接传入id
        //id的唯一，只选择到了第一个匹配的id为imooc的div节点
        $('#imooc').css("border", "3px solid red");
    </script>

</body>

</html>